<!DOCTYPE html>
<style>
.container {
  margin: 4px;
  display: inline-block;
  border: 1px solid blue;
}
.vscroller {
  overflow-y: overlay;
  max-height: 300px;
  border: 2px solid black;
  padding: 2px;
  background-color: white;
}
.item-container {
  display: flex;
  flex-direction: column;
  justify-content: start;
  contain: paint;
}
.item {
  font-size: 18px;
  flex: 0;
  margin: 1px 0;
}
</style>

<body></body>

<script src='../resources/runner.js'></script>
<script src='../resources/generate-chrome-class-name.js'></script>
<script>

// To generate profile data for analysis with pprof, set generateProfile to
// 'true' and run chrome with "--enable-gpu-benchmarking --no-sandbox".
const generateProfile = false;

// How many observed list items.
const numItems = 100000;
// How many levels of DOM hierarchy between the scrolling root and list items.
const containerDepth = 100;
// How far to scroll on each frame.
const velocity = 1000;

let scroller = document.createElement('div');
scroller.classList.add('vscroller');
document.body.appendChild(scroller);
let parent = scroller;
for (let i = 0; i < containerDepth; i++) {
  let container = document.createElement('div');
  container.classList.add('container');
  parent.appendChild(container);
  parent = container;
}
let itemContainer = document.createElement('div');
itemContainer.classList.add('item-container');
parent.appendChild(itemContainer);
parent = itemContainer;

let observer = new IntersectionObserver(entries => {
  entries.forEach(entry => {
    if (entry.intersectionRatio > 0) {
      entry.target.classList.add('visible');
    } else {
      entry.target.classList.remove('visible');
    }
  });
}, { root: scroller });

for (let i = 0; i < numItems; i++) {
  let item = document.createElement('div');
  item.classList.add('item');
  parent.appendChild(item);
  item.innerText = generateChromeClassName();
  observer.observe(item);
}

PerfTestRunner.measureFrameTime({
  description: 'Frame time with one IntersectionObserver with a scrolling root element, observing many items.',
  tracingCategories: 'blink',
  traceEventsToMeasure: ['LocalFrameView::UpdateViewportIntersectionsForSubtree'],
  setup: () => {
    let max_scroll = scroller.scrollHeight - scroller.clientHeight;
    scroller.scrollTop = (scroller.scrollTop + velocity) % max_scroll;
    if (generateProfile && self.chrome && chrome.gpuBenchmarking) {
      chrome.gpuBenchmarking.startProfiling('perf.data');
    }
  },
  run: () => {},
  done: () => {
    if (generateProfile && self.chrome && chrome.gpuBenchmarking) {
      chrome.gpuBenchmarking.stopProfiling();
    }
  }
});

</script>
